There's no way to anticipate the thought processes of Democrat Obama and Republican John McCain, no formula for weighing the factors involved.

And this year, as ever, lessons learned seem to have been forgotten.

Consider that much of the speculation has centered on individuals presumed to be able to deliver their states in November - even though few running mates have delivered states before, and current polls suggest it's not likely to happen in 2008, either.

Obama and McCain could announce their picks at any time between now and the late-summer conventions. History suggests later rather than sooner; nominees tend to want to weigh their options as long as possible.

In the end, of course, few Americans will vote on the basis of the vice president. But the selection of No. 2 says a lot about the thinking of No. 1. "Vice presidential candidates are in the spotlight the week they're chosen and the week of their debate," said Michael Nelson, a political scientist at Rhodes College in Tennessee who has studied the subject. "That's a pretty good chunk of the campaign."

In addition, recent years have demonstrated how much it matters who sits in the vice president's office, beyond the right of succession.

One indicator of how the process may play out this time is how it has played out in the past. What follows are historical trends. And trends, even more than rules, are made to be broken.

Governors need not apply.

The last to make the cut was Republican Spiro Agnew, who ran with Richard Nixon in 1968 and 1972. Agnew wound up resigning and going to prison. Since then, the major parties have looked in other directions.

One reason is that a lot of presidential nominees (Jimmy Carter, Ronald Reagan, Bill Clinton, George W. Bush) had been governors. So they tended to want Washington figures running with them.

"Outsiders pick insiders," said Joel Goldstein, an expert on the vice presidency at St. Louis University. "Insiders usually pick insiders, too."

This time, though, with two senators running for president, governors may actually have a chance.

Also-rans from the nomination fight don't win the prize.

From time to time, a presidential nominee has selected the runner-up. Democrat John F. Kennedy went with Lyndon B. Johnson in 1960, Republican Reagan with George H.W. Bush in 1980, Democrat John Kerry with John Edwards in 2004.

But no candidate has passed over the runner-up to select a straggler. This does not augur well for New Mexico Gov. Bill Richardson, Sen. Joseph R. Biden Jr. of Delaware, and Sen. Christopher J. Dodd of Connecticut, all of whom finished far behind Hillary Clinton.

Running mates aren't picked for their ability to carry their home state. Or shouldn't be.

To be sure, Johnson helped carry Texas for Kennedy in 1960, although the Texan was chosen for his appeal to the entire South. Since then, perhaps the only running mate who helped win a state that a ticket otherwise might have lost was Al Gore (Tennessee) with Bill Clinton in 1992.

According to a 1989 study, a running mate from a state improves the ticket's performance there by an average of 0.3 percentage points.

This month, pollsters from Quinnipiac University asked Floridians about having Republican Gov. Charlie Crist or Democratic Sen. Bill Nelson as vice presidential candidates; Pennsylvanians about former Gov. Tom Ridge, a Republican, and Gov. Rendell; Ohioans about former Rep. Rob Portman, a Republican, and Democratic Gov. Ted Strickland.

Ridge polled the best: He had no impact. All the others made voters less likely to support Obama or McCain.

That hasn't stopped the talk about politicians with presumed local appeal, including Republican Gov. Tim Pawlenty of Minnesota and two Virginia Democrats, Sen. Jim Webb and Gov. Tim Kaine.

With his choice, the presidential nominee will make a statement of some sort.

By picking Dick Cheney in 2000, George W. Bush said he was confident enough of himself - and aware of his own inexperience - to go with a strong Washington insider.

Obama, who has little foreign-policy experience, might be inclined to pick someone with strong credentials in that realm. Rendell has recommended Hillary Clinton, Biden or Sen. Evan Bayh of Indiana.

McCain might be drawn to someone with a business background, such as former Massachusetts Gov. Mitt Romney. Or he might pick a woman, particularly if Obama doesn't. At 71, McCain knows that his choice will draw close scrutiny, particularly if he opts for a much younger person with limited experience. Said Goldstein: "In picking a running mate, good government is good politics. If the person doesn't pass the test of being presidential, it hurts your credibility. It's a drop-dead requirement."

The announcements probably won't happen soon.

They used to come at the conventions. More recently, they have usually been announced the week before, giving delegates time to deal with any anger or disappointment before arriving on site.

The exception was Kerry. He announced Edwards, a move he knew would be popular, 20 days before the 2004 Democratic convention.

With the Democrats meeting first this year, as the out-party always does, McCain has the opportunity to wait out Obama, then factor Obama's move into his own calculus.

In terms of the outcome in November, the choice probably won't be decisive.

With the possible exception of 1960, it's fair to say that there hasn't been an election where a vice presidential candidate was the difference between victory and defeat.

But analysts say three running mates damaged their tickets - Agnew, Dan Quayle and Democratic Rep. Geraldine Ferraro. The GOP won despite Agnew in 1968 and Quayle 20 years later; the Democrats would have lost in 1984 no matter whom Walter Mondale chose as his No. 2.

In the end, few politicians refuse to be No. 2.

In 1972, presidential nominee George McGovern, a prohibitive underdog, was turned down by several prominent Democrats before settling on Sen. Thomas Eagleton of Missouri, who then had to leave the ticket after revelations that he'd been treated for depression.

That's the exception, especially now that the vice presidency has become an office with real clout. "Unlike most other people, I'm being straight with you," Biden said on NBC's Meet the Press this month, after saying he did not want the job. "If asked, I will do it."